The present invention is related LCD panels and displays, and, more particularly, to a circuit and method for adjusting the “VCOM” level associated with the LCD panel.
As is well known in the art, the VCOM or common voltage of an LCD panel needs to be adjusted to remove undesirable flicker from the display. Various solutions to adjusting the VCOM voltage have been proposed in the prior art.
Ideally, the VCOM output should be adjusted as a certain value in the range from (⅓)*AVDD to (⅔)*AVDD (wherein AVDD is the main analog voltage supply for the LCD). The exact value necessary to remove flicker from the LCD display panels will vary according to different LCD manufacturers. If the actual VCOM output value deviates from the desired VCOM value, the LCD will flicker. The intensity of the flicker will increase the more the VCOM value deviates from the optimum desired value. The relationship between VCOM level distribution and LCD flicker intensity is shown in FIG. 1. Consequently, a user can readily determine the optimum VCOM level according to the flicker intensity of the LCD display.
FIG. 2 shows two types of VCOM calibrators 200 and 202, each using mechanical potentiometers. The circuit includes external resistor Rd and Rf as well as mechanical potentiometer that includes adjustable resistor Re. Resistors Rd, Re, and Rf are coupled between AVDD and ground, and a buffer coupled to the output of the potentiometer resistor Re. The output of the buffer provides the VCOM output signal for the LCD display. In circuit 200, the output of the potentiometer is fed directly into the positive input of the buffer. In circuit 202, the output of the potentiometer is coupled to the junction of resistors Re and Rf, and then fed into the positive input of the buffer. In circuit 200, the VCOM level output is given by:
                                                                        V                ⁢                                                                  ⁢                COM                            =                                                AVDD                  *                                      (                                          Rf                      +                                              x                        *                        Re                                                              )                                                                    Rd                  +                  Re                  +                  Rf                                                                                        x              ∈                              [                                  0                  ,                  1                                ]                                                                        (        1        )            In circuit 202, the VCOM level output is:
                                                                        V                ⁢                                                                  ⁢                COM                            =                                                AVDD                  *                  Rf                                                  Rd                  +                                                            (                                              1                        -                        x                                            )                                        *                    Re                                    +                  Rf                                                                                        x              ∈                              [                                  0                  ,                  1                                ]                                                                        (        2        )            As we can be seen from a review of equations (1) and (2), the VCOM level output can be changed by adjusting the resistive value of Re accordingly.
There are several drawbacks of using mechanical potentiometers to set the VCOM voltage. The resolution and reliability are relatively poor for a mechanical potentiometer. It is time consuming and labor intensive to mass produce LCD panels using mechanical potentiometers. Further, it is difficult to realize factory automation using mechanical potentiometers. To address these problems, panel manufacturers have started to use digital VCOM calibrators, which greatly simplifies the adjustment process and increases reliability.
FIG. 3 shows the implementation of a VCOM calibrator 300 with an external resistive voltage-divider. Calibrator 300 includes an interface and control block 302, a bus 304 for communicating with a single DAC (“Digital to Analog Converter”) 308, and a bus 306 for communicating with non-volatile memory block 310. The output of DAC 308 is coupled to the positive input of buffer 312, which drives the gate of transistor M1. The source of transistor M1 is coupled to the RSET resistor at the SET node. The drain of transistor M1 is coupled to the output of an external resistor divider at the OUT node. The external resistor divider includes resistors R1 and R2, which is coupled between the AVDD power supply and ground. The OUT node is buffered through buffer 314 to provide the VCOM voltage.
In FIG. 3, the interface and control logic block 302 are used to manage digital signals such as clock, data in, data out, address and control signals. Non-volatile memories (NVMEM) 310 are used to store values of the desired VCOM level. The single DAC 308 is used to adjust the VCOM level. R1, R2 and RSET are external resistors that determine the adjustment range for the VCOM output.
The basic principle for the VCOM calibrator 300 with an external voltage-divider is as follows. The external resistive voltage-divider comprising R1 and R2 sets the maximum value of the VCOM adjustment range. Resistor RSET sets the full-scale sink current, IOUT, which determines the minimum value of the VCOM adjustment range. A larger RSET value increases the resolution but decreases the VCOM adjustment range. The value of resistors R1, R2 and RSET can be calculated by using the following procedure.    1) Set an approximate adjustment range that contains the desired VCOM level. Set the maximum VCOM level as VMAX and the minimum VCOM level as VMIN.    2) Calculate R1/R2 ratio as:
                                          R            ⁢                                                  ⁢            1                                R            ⁢                                                  ⁢            2                          ≈                              AVDD                          V              MAX                                -          1                                    (        3        )                3) Calculate R1/RSET ratio as:
                                          R            ⁢                                                  ⁢            1                                R            SET                          ≈                              (                                                            V                  MAX                                                  V                  MIN                                            -              1                        )                    *          Y                                    (        4        )            where Y=the number of resistors R in DAC.    4) Choose RSET according to the electrical limitations.    5) Calculate R1 and R2.    6) Change the DAC value from 0 to 2n−1 (n is the bit number of DAC) to adjust the VCOM level between VMIN and VMAX until the optimum level is reached.The range for the adjustment is given by:Range=VMAX−VMIN   (5)and the resolution is given by:
                    Resolution        =                                            V              MAX                        -                          V              MIN                                                          2              n                        -            1                                              (        6        )            Where n is the resolution of DAC.
From the description above, there are some drawbacks to this method.    1) Since there is a tradeoff between the adjustment range and the resolution, the adjustment range is generally a small part of the overall possible range of VCOM distribution (generally from ⅓*AVDD to ⅔*AVDD, where AVDD is the analog voltage supply). Thus, it is firstly necessary to test and gain an approximate range that contains the desired VCOM level.    2) After the approximate range is found, it is also necessary to calculate the external resistors R1, R2 and RSET that determine the range.    3) The external resistors are, in general, less reliable and occupy PCB (“printed circuit board”) area compared with integrated resistors.
What is desired, therefore, is a VCOM calibrator circuit for an LCD that can provide an optimum VCOM adjustment voltage, but does not use external resistors or potentiometers.